Objective lens for optical recording media and optical pickup apparatus using the same

ABSTRACT

An objective lens for optical recording media is constituted by two or three lenses cemented together and used for recording or reproducing two kinds of optical recording media having respective thicknesses different from each other with two light beams having wavelengths λ 1  and λ 2  different from each other. The cemented face exhibits negative and positive refracting powers with respect to light beams having wavelengths λ 1  and λ 2 , respectively, according to the wavelength-dependent difference in refractive index of materials constituting the respective lenses.

RELATED APPLICATIONS

This application claims the priority of Japanese Patent Application No.2000-245194 filed on Aug. 11, 2000, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an objective lens and optical pickupapparatus for recording and reproducing signals, which are commonlyusable for two or more kinds of optical recording media havingthicknesses different from each other. In particular, the presentinvention relates to an objective lens which, when light beams forirradiating the optical recording media have wavelengths different fromeach other according to the kinds of optical recording media, canefficiently converge the respective light beams onto their correspondingoptical recording media; and an optical pickup apparatus using the same.

2. Description of the Prior Art

In recent years, various kinds of optical recording media have beenunder development, and optical pickup apparatus which can carry outrecording and reproducing while using a plurality of kinds of opticalrecording media in common have been known. For example, a system whichcarries out recording and reproducing of DVD (digital versatile disc)and CD-R (recordable optical disc) by using a single optical pickupapparatus has been known.

In such two kinds of optical recording media, for example, visible lightat about 650 nm is used for DVD in order to improve the recordingdensity, whereas near-infrared light at about 780 nm is required to beused for CD-R since it has no sensitivity for light in the visibleregion. An optical pickup apparatus which can be used in common for bothof them is based on a dual-wavelength beam type which uses two lightbeams having wavelengths different from each other.

In the two optical recording media exemplified above, however, the discthickness of the CD-R is fixed to 1.2 mm whereas the DVD in use mostlyhas a thickness of 0.6 mm. Therefore, the individual wavelengths oflight for reproducing or recording are needed to be securely convergedat their predetermined positions different from each other.

Under such circumstances, a system in which two objective lenses havingconverging actions different from each other are made switchabledepending on the optical recording media subjected to reproducing orrecording has been known. However, it complicates the structure ofoptical pickup apparatus and opposes the demands for compactness andlower cost.

On the other hand, it has recently been known to use a multifocal lenscomposed of a diffractive optical element or a plurality of circularstrip divisions for an objective lens part of an optical pickupapparatus (Japanese Unexamined Patent Publication No. 8-62493, JapaneseUnexamined Patent Publication No. 9-145994, and the like).

In the apparatus using the above-mentioned diffractive optical element,however, a noise light component of an order different from thatcontributing to the recording and reproducing is emitted from thediffractive optical element. In the apparatus using the above-mentionedmultifocal lens, the light transmitted through a circular strip divisionother than that contributing to the recording and reproducing at thattime is noise light. Due to these noise light components, the S/N ratioat the recording and reproducing deteriorates.

SUMMARY OF THE INVENTION

In view of the circumstances mentioned above, it is an object of thepresent invention to provide a compact, inexpensive objective lens witha favorable S/N ratio for optical recording media, which can favorablyconverge two wavelengths of light onto their corresponding recordingsurfaces of optical recording media having respective thicknesseswithout complicating the structure of optical pickup apparatus; and anoptical pickup apparatus using the same.

The present invention provides an objective lens for optical recordingmedia, the objective lens being constituted by two lenses cementedtogether and used for recording or reproducing two kinds of opticalrecording media having thicknesses different from each other with twolight beams having wavelengths different from each other, respectively;

the objective lens satisfying the following conditional expression (1)with respect to a light beam having a wavelength λ₁ used for recordingor reproducing one of the two optical recording media, and

the following conditional expression (2) with respect to a light beamhaving a wavelength λ₂ used for recording or reproducing the other ofthe two optical recording media:

N_(Lλ1) >N _(Dλ1)  (1)

N_(Lλ2) <N _(Dλ2)  (2)

 where

N_(Lλ1) is the refractive index of the lens on the light source sidewith respect to the light beam having the wavelength λ₁;

N_(Dλ1) is the refractive index of the lens on the optical recordingmedia side with respect to the light beam having the wavelength λ₁;

N_(Lλ2) is the refractive index of the lens on the light source sidewith respect to the light beam having the wavelength λ₂; and

N_(Dλ2) is the refractive index of the lens on the optical recordingmedia side with respect to the light beam having the wavelength λ₂.

Also, the present invention provides an objective lens for opticalrecording media, the objective lens being constituted by two lensescemented together and used for recording or reproducing two kinds ofoptical recording media having thicknesses different from each otherwith two light beams having wavelengths different from each other,respectively;

the objective lens satisfying the following conditional expression (3)with respect to a light beam having a wavelength λ₁ used for recordingor reproducing one of the two optical recording media, and

the following conditional expression (4) with respect to a light beamhaving a wavelength λ₂ used for recording or reproducing the other ofthe two optical recording media:

N_(Lλ1)<N_(Dλ1)  (3)

N_(Lλ2)>N_(Dλ2)  (4)

 where

N_(L) _(^(λ)) ₁ is the refractive index of the lens on the light sourceside with respect to the light beam having the wavelength λ₁;

N_(D) _(^(λ)) ₁ is the refractive index of the lens on the opticalrecording media side with respect to the light beam having thewavelength λ₁;

N_(L) _(^(λ)) ₂ is the refractive index of the lens on the light sourceside with respect to the light beam having the wavelength λ₂; and

N_(D) _(^(λ)) ₂ is the refractive index of the lens on the opticalrecording media side with respect to the light beam having thewavelength λ₂.

Further, the present invention provides an objective lens for opticalrecording media, the objective lens being constituted by two lensescemented together and used for recording or reproducing two kinds ofoptical recording media having thicknesses different from each otherwith two light beams having wavelengths different from each other,respectively;

assuming that the optical recording medium recorded or reproduced with alight beam having a wavelength λ₁ and the optical recording mediumrecorded or reproduced with a light beam having a wavelength λ₂ haverespective thicknesses t₁ and t₂ satisfying the condition of t₁<t₂,

the objective lens satisfying the following conditional expression (5)when the cemented face of the cemented lenses is convex toward the lightsource, and

the following conditional expression (6) when the cemented face of thecemented lenses is convex toward the optical recording media:

N_(Lλ1)>N_(Dλ1) and N_(Lλ2)<N_(Dλ)2  (5)

 N_(Lλ1)<N_(Dλ1) and N_(Lλ2)>N_(Dλ2)  (6)

 where

N_(L) _(^(λ)) ₁ is the refractive index of the lens on the light sourceside with respect to the light beam having the wavelength λ₁;

N_(D) _(^(λ)) ₂ is the refractive index of the lens on the opticalrecording media side with respect to the light beam having thewavelength λ₁;

N_(L) _(^(λ)) ₂ is the refractive index of the lens on the light sourceside with respect to the light beam having the wavelength λ₂; and

N_(D) _(^(λ)) ₂ is the refractive index of the lens on the opticalrecording media side with respect to the light beam having thewavelength λ₂.

Furthermore, the present invention provides an objective lens foroptical recording media, the objective lens being constituted by threelenses cemented together and used for recording or reproducing two kindsof optical recording media having thicknesses different from each otherwith two light beams having wavelengths different from each other,respectively;

assuming that the optical recording medium recorded or reproduced with alight beam having a wavelength λ₁ and the optical recording mediumrecorded or reproduced with a light beam having a wavelength λ₂ haverespective thicknesses t₁ and t₂ satisfying the condition of t₁<t₂,

the objective lens satisfying the following conditional expression (7)when the cemented face between the first and second lenses from thelight source side is convex toward the light source:

N_(1λ1)>N_(2λ1) and N_(1λ2)<N_(2λ2)  (7)

the objective lens satisfying the following conditional expression (8)when the cemented face between the first and second lenses from thelight source side is convex toward the optical recording media:

N_(1λ1)<N_(2λ1) and N_(1λ2)>N_(2λ2)  (8)

the objective lens satisfying the following conditional expression (9)when the cemented face between the second and third lenses from thelight source side is convex toward the light source:

N_(2λ1)>N_(3λ1) and N_(2λ2)<N_(3λ2)  (9)

the objective lens satisfying the following conditional expression (10)when the cemented face between the second and third lenses from thelight source side is convex toward the optical recording media:

N_(2λ1)<N_(3λ1) and N_(2λ2)>N_(3λ2)  (10)

 where

N₁ _(^(λ)) ₁ is the refractive index of the first lens from the lightsource side with respect to the light beam having the wavelength λ₁;

N₂ _(^(λ)) ₁ is the refractive index of the second lens from the lightsource side with respect to the light beam having the wavelength λ₁;

N₃ _(^(λ)) ₁ is the refractive index of the third lens from the lightsource side with respect to the light beam having the wavelength λ₁;

N₁ _(^(λ)) ₂ is the refractive index of the first lens from the lightsource side with respect to the light beam having the wavelength λ₂;

N₂ _(^(λ)) ₂ is the refractive index of the second lens from the lightsource side with respect to the light beam having the wavelength λ₂; and

N₃ _(^(λ)) ₂ is the refractive index of the third lens from the lightsource side with respect to the light beam having the wavelength λ₂.

Preferably, the objective lens in this case satisfies the followingconditional expressions (11) and (12):

N_(1λ1)=N_(3λ1)  (11)

N_(1λ2)=N_(3λ2)  (12)

Preferably, the first and third lenses from the light source side areformed from the same material.

The cemented faces of the lenses may be made aspheric.

In each of the above-mentioned objective lenses for optical recordingmedia, one of the light beams having wavelengths λ₁ and λ₂ may be alight beam having a wavelength of 650 nm used for recording orreproducing DVD, whereas the other may be a light beam having awavelength of 780 nm used for recording or reproducing CD-R.

The present invention provides an optical pickup apparatus comprisingany of the above-mentioned objective lens for optical recording media.

In general, objective lenses for optical discs commonly used forrecording/reproducing various kinds of discs having respectivethicknesses different from each other have the following problems ingeneral.

Namely, if an objective lens designed for a relatively thick disc isused for recording/reproducing a relatively thin disc, a negativespherical aberration will occur, whereby recording/reproducing cannot beeffected sufficiently.

If an objective lens designed for a relatively thin disc is used forrecording/reproducing a relatively thick disc, by contrast, a positivespherical aberration will occur, whereby recording/reproducing cannot beeffected sufficiently.

For example, if an objective lens designed for CD-R (having a thicknessof 1.2 mm) is used for DVD (having a thickness of 0.6 mm), a negativespherical aberration will occur, whereby recording/reproducing cannot beeffected sufficiently.

If an objective lens designed for DVD (having a thickness of 0.6 mm) isused for CD-R (having a thickness of 1.2 mm), by contrast, a positivespherical aberration will occur, whereby recording/reproducing cannot beeffected sufficiently.

Therefore, when a lens is to be designed such that both of theabove-mentioned thick and thin discs can be recorded/reproduced, lensdesign parameters are required to be such that the spherical aberrationchanges in opposite directions depending on whether the above-mentionedthick or thin disc is used.

When a lens surface is convex onto the air side at the boundary betweenthe lens and air, i.e., it is a lens surface having a positiverefracting power, the spherical aberration will change in the negativedirection as the curvature of the lens surface increases in general.

When the lens surface is concave onto the air side, i.e., it is a lenssurface having a negative refracting power, by contrast, the sphericalaberration will change in the positive direction as the curvature of thelens surface increases in general.

A lens constituted by a single group having a single elementconfiguration has only boundary faces in contact with the air.Therefore, as can be seen from the foregoing, this lens has no lenssurface which can change the spherical aberration in both of thepositive and negative directions at the same time when the curvature ofboundary faces is changed.

By contrast, a cemented lens can have either a positive or negativerefracting power due to the difference in magnitude of refractiveindices of materials in front and rear of the cemented face even whenthe surface form is the same.

Namely, when the cemented face is convex toward the light source, forexample, if the lens material positioned on the light source side of thecemented face has a refractive index lower than that of the lensmaterial positioned on the optical recording media side of the cementedface, then the cemented face can be formed as a lens surface having apositive refracting power. If the lens material positioned on the lightsource side of the cemented face has a refractive index higher than thatof the lens material positioned on the optical recording media side ofthe cemented face, by contrast, then the cemented face can be formed asa lens surface having a negative refracting power.

Thus, the cemented face can be made as a lens surface having a positiverefracting power or a lens surface having a negative refracting poweraccording to the difference in refractive index on the front and rearsides thereof even when the surface form is the same. When the curvatureof the cemented face is made large, the spherical aberration can bechanged in the negative or positive direction according to therefracting power of the lens surface at this time.

Taking account of such a property of the cemented face, the presentinvention is achieved.

When the wavelength of light employed varies depending on the discthickness, the wavelength-dependent difference in refractive index ofthe material constituting the lens due to the above-mentioned propertyof the cemented face is utilized so as to exhibit a negative refractingpower (or positive refracting power) and a positive refracting power (ornegative refracting power) with respect to the light beams havingwavelengths λ₁ and λ₂, respectively.

For example, suppose a case where a cemented lens is used as anobjective lens for an optical disc for recording/reproducing discshaving thicknesses of t₁ and t₂ (t₁<t₂) with light beams havingwavelengths λ₁ and λ₂, respectively, whereas its cemented face is convextoward the light source. In this case, if the lens material on the lightsource side of the cemented face has a refractive index higher than thatof the lens material on the optical recording media side with respect tothe light beam having the wavelength λ₁ and has a refractive index lowerthan that of the lens material on the optical recording media side withrespect to the light beam having the wavelength λ₂, then the cementedface can exhibit negative and positive refracting powers with respect tothe light beams having wavelengths λ₁ and λ₂, respectively.

As the curvature of the cemented face is enhanced, the sphericalaberration can be changed in the positive direction with respect to thelight beam having the wavelength λ₁, and in the negative direction withrespect to the light beam having the wavelength λ₂. Thus, lens designparameters can exist such that the spherical aberration changes inopposite directions depending on whether a relatively thick or thin discis used, thereby making it possible to construct an objective lens whichcan record/reproduce discs having thicknesses different from each other.

When the cemented face is convex toward the optical recording media inthe above-mentioned example, effect similar to those mentioned above canbe obtained if the relationship between the refractive indices of lensmaterials is in the reverse of that mentioned above.

Though Japanese Unexamined Patent Publication No. 10-332906 and JapanesePatent Publication No. 2727373 disclose cemented objective lenses foroptical pickups, each of them is assumed to use a single wavelength oflight, whereby they are hard to exhibit the effects of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic views showing the states where theobjective lens in accordance with Example 1 of the present invention isused for DVD and CD-R, respectively;

FIGS. 2A and 2B are schematic views showing the states where theobjective lens in accordance with Example 2 of the present invention isused for DVD and CD-R, respectively;

FIGS. 3A and 3B are schematic views showing the states where theobjective lens in accordance with Example 3 of the present invention isused for DVD and CD-R, respectively;

FIGS. 4A and 4B are schematic views showing the states where theobjective lens in accordance with Example 4 of the present invention isused for DVD and CD-R, respectively;

FIGS. 5A and 5B are schematic views showing the states where theobjective lens in accordance with Example 5 of the present invention isused for DVD and CD-R, respectively;

FIGS. 6A and 6B are aberration charts showing wavefront aberrations inExample 1 of the present invention;

FIGS. 7A and 7B are aberration charts showing wavefront aberrations inExample 2 of the present invention;

FIGS. 8A and 8B are aberration charts showing wavefront aberrations inExample 3 of the present invention;

FIGS. 9A and 9B are aberration charts showing wavefront aberrations inExample 4 of the present invention;

FIGS. 10A and 10B are aberration charts showing wavefront aberrations inExample 5 of the present invention; and

FIG. 11 is a schematic view showing the optical pickup apparatus inaccordance with an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, an embodiment of the present invention will beexplained with reference to the drawings.

First, with reference to FIG. 11, an optical pickup apparatus using theobjective lens for optical recording media in accordance with anembodiment of the present invention will be explained.

In this optical pickup apparatus, a semiconductor laser 11B and asemiconductor laser 11C output a laser beam 12 when power is suppliedthereto from an LD power source 11A, a half mirror 13 reflects the laserbeam 12, a collimator lens 4 turns thus reflected laser beam 12 intoparallel light, and an objective lens 5 converts the parallel light intoconvergent light, with which a recording region 6P of an optical disc 6is irradiated. The semiconductor laser 11B is a light source foroutputting a laser beam in a near-infrared region having a wavelength ofabout 780 nm for CD-R (recordable optical disc), whereas thesemiconductor laser 11C is a light source for outputting a laser beam,for example, in a visible region having a wavelength of 650 nm for DVD(digital versatile disc). The laser beam 12 outputted from one of thesemiconductor lasers 11B, 11C irradiates the half mirror 13 by way of ahalf mirror 11D. A changeover switch 11E is disposed between the LDpower source 11A and the semiconductor lasers 11B, 11C. As thechangeover switch 11E is operated, power is supplied to one of thesemiconductor lasers 11B, 11C.

In the recording region 6P, pits carrying signal information arearranged in a track. The reproducing reflected light of laser beam 12from the recording region 6P is made incident on the half mirror 13 byway of the objective lens 5 and collimator lens 4 while carrying thesignal information, and is transmitted through the half mirror 13, so asto be made incident on a four-part photodiode 7. The respectivequantities of light received at the four separate diode positions arearithmetically operated in this photodiode 7, whereby data signals andrespective error signals for focusing and tracking are obtained.

Since the half mirror 13 is inserted in the optical path of the returnlight from the optical disc 6 in a state tilted by 45°, it acts like acylindrical lens, so that the light beam transmitted through the halfmirror 13 has an astigmatism, whereby the amount of focusing error isdetermined according to the form of the beam spot of return light on thefour-part photodiode 7. Here, the collimator lens 4 can be omitteddepending on the circumstances. Also, a grating may be inserted betweenthe semiconductor lasers 11B, 11C and the half mirror 13, such thattracking errors can be detected by use of three beams.

Thus, the optical pickup apparatus in accordance with this embodiment isconfigured such that signals can be recorded and reproduced for anyoptical disc 6 of CD-R and DVD.

Here, each of the CD-R and DVD has a protective sheet made of PC(polycarbonate).

Meanwhile, the geometric thickness of CD-R is standardized at 1.2 mm,and one having a refractive index of 1.55 is often used. As for the DVD,on the other hand, one having a geometric thickness of 0.6 mm and arefractive index of 1.58 is often used. Consequently, for securelycarrying out focusing on each optical disc 6, it is necessary to providea configuration yielding focusing actions different from each other forthe respective wavelengths of light for recording/reproducing thedifferent kinds of discs.

Therefore, the above-mentioned optical pickup apparatus is provided withthe objective lens 5 in which a plurality of lenses 1, 2 whoserefractive index changing ratios differ from each other according towavelengths are cemented together as shown in FIG. 11, so that both CD-Rand DVD can be recorded and reproduced.

When a DVD is disposed at a predetermined position (on a turntable) forrecording or reproducing, the laser beam 12 having a wavelength of 650nm (λ₁) from the semiconductor laser 11C is made incident on theobjective lens 5 while in a substantially parallel state. In this case,the incident laser beam 12 is converged onto a recording surface of theDVD by the objective lens 5.

When a CD-R is disposed at a predetermined position (on the turntable)for recording or reproducing, the laser beam 12 having a wavelength of780 nm (λ₂) from the semiconductor laser 11B is made incident on theobjective lens 5 while in a substantially parallel state. In this case,the incident laser beam 12 is converged onto a recording surface of theCD-R by the objective lens 5.

Here, in the objective lens 5, the lens on the light source side and thelens on the optical recording media side in the cemented lenses haverespective refractive index differences in polarities different fromeach other according to wavelengths.

The above-mentioned objective lens 5 will now be explained specificallywith reference to Examples 1 to 5.

EXAMPLES Example 1

When a DVD 6A is disposed at a predetermined position (on a turntable)for recording or reproducing as shown in FIG. 1A, the laser beam 12having a wavelength of 650 nm (λ₁) is made incident on the objectivelens 5 while in a substantially parallel state. In this case, theincident laser beam 12 is converged onto a recording surface of the DVD6A by the objective lens 5.

When a CD-R 6B is disposed at a predetermined position (on theturntable) for recording or reproducing as shown in FIG. 1B, on theother hand, the laser beam 12 having a wavelength of 780 nm (λ₂) is madeincident on the objective lens 5 while in a substantially parallelstate. In this case, the incident laser beam 12 is converged onto arecording surface of the CD-R 6B by the objective lens 5.

The objective lens 5 in Example 1 comprises, successively from the lightsource side, a negative meniscus lens 2 having a convex surfaceconstituted by an aspheric surface (represented by the followingaspheric surface expression, ditto for the following aspheric surfaces)directed onto the light source side and a biconvex lens 1 having asurface with a weaker curvature constituted by an aspheric surfacedirected onto the optical recording media side, which are cementedtogether. Here, the cemented face is an aspheric surface. Since bothsides and cemented face of the objective lens 5 are constituted byaspheric surfaces, the freedom in designing of aberration correction andthe correcting effect can be improved greatly.

In this Example, the cemented face is made convex toward the lightsource, the refractive index of the lens 2 on the light source side ofthe cemented face is made higher than that of the lens 1 on the opticalrecording media side with respect to the light beam having thewavelength λ₁, and is made lower than that of the lens 1 on the opticalrecording media side with respect to the light beam having thewavelength λ₂, whereby the cemented face can have negative and positiverefracting powers with respect to the light beams having the wavelengthsλ₁ and λ₂, respectively.

When the curvature of the cemented face is enhanced, the sphericalaberration can be changed in the positive and negative directions withrespect to the light beams having the wavelengths λ₁ and λ₂,respectively. Thus, lens design parameters can exist such that thespherical aberration changes in directions opposite from each otherdepending on whether the CD-R 6B, which is a relatively thick disc, orthe DVD 6A, which is a relatively thin disc, is used. Hence, theobjective lens 5 having a small spherical aberration commonly used forDVD and CD-R can be realized.$Z = {\frac{Y^{2}/R}{1 + \left( {1 - {{KY}^{2}/R^{2}}} \right)^{1/2}} + {\sum\limits_{i = 2}^{5}\quad {A_{i}Y^{2i}}}}$

where

Z is the length of the perpendicular to a tangential plane (planeperpendicular to the optical axis) of an apex of the aspheric surfacefrom a point on the aspheric surface having a distance Y from theoptical axis;

Y is the distance from the optical axis;

A_(i) is the aspheric surface coefficient (i=2 to 5);

R is the radius of curvature of aspheric surface near the optical axis;and

K is the eccentricity.

The upper part of Table 1 (follows) shows lens data (radius of curvatureR, surface space D, and refractive index N with respect to λ=650 nm and780 nm) of the objective lens 5 in accordance with Example 1.

The middle part of Table 1 indicates the aspheric surface coefficient ofeach aspheric surface.

The lower part of Table 1 indicates the focal length of objective lens5, the diameter of luminous flux incident on the objective lens 5, andthe numerical aperture of objective lens 5 in each of the cases whereDVD and CD-R are set as the optical recording medium.

Example 2

The objective lens 5 in Example 2 comprises, successively from the lightsource side, a biconvex lens 1 a having a surface with a weakercurvature constituted by an aspheric surface directed onto the lightsource side and a negative meniscus lens 2 a having a convex surfaceconstituted by an aspheric surface directed onto the optical recordingmedia side, which are cemented together. Here, the cemented face is anaspheric surface.

In this Example, the cemented face is made convex toward the opticalrecording media, the refractive index of the lens 1 a on the lightsource side of the cemented face is made lower than that of the lens 2 aon the optical recording media side with respect to the light beamhaving the wavelength λ₁, and is made higher than that of the lens 2 aon the optical recording media side with respect to the light beamhaving the wavelength λ₂, whereby the cemented face can have negativeand positive refracting powers with respect to the light beams havingthe wavelengths λ₁ and λ₂, respectively.

When the curvature of the cemented face is made greater, the sphericalaberration can be changed in the positive and negative directions withrespect to the light beams having the wavelengths λ₁ and λ₂,respectively. Thus, lens design parameters can exist such that thespherical aberration changes in directions opposite from each otherdepending on whether the CD-R 6B, which is a relatively thick disc, orthe DVD 6A, which is a relatively thin disc, is used. Hence, theobjective lens 5 having a small spherical aberration commonly used forDVD and CD-R can be realized.

The upper part of Table 2 (follows) shows lens data (radius of curvatureR, surface space D, and refractive index N with respect to λ=650 nm and780 nm) of the objective lens 5 in accordance with Example 2.

The middle part of Table 2 indicates the aspheric surface coefficient ofeach aspheric surface.

The lower part of Table 2 indicates the focal length of objective lens5, the diameter of luminous flux incident on the objective lens 5, andthe numerical aperture of objective lens 5 in each of the cases whereDVD and CD-R are set as the optical recording medium.

Example 3

The objective lens 5 in Example 3 comprises, successively from the lightsource side, a negative meniscus lens 2 b having an aspheric convexsurface directed onto the light source side and a biconvex lens 1 bhaving a surface with a weaker curvature constituted by an asphericsurface directed onto the optical recording media side, which arecemented together. Here, the cemented face is a spherical surface. Sinceboth sides of the objective lens 5 are constituted by aspheric surfaces,the freedom in designing of aberration correction and the correctingeffect can be improved greatly.

The effects of this Example are substantially the same as those ofExample 1.

The upper part of Table 3 (follows) shows lens data (radius of curvatureR, surface space D, and refractive index N with respect to λ=650 nm and780 nm) of the objective lens 5 in accordance with Example 3.

The middle part of Table 3 indicates the aspheric surface coefficient ofeach aspheric surface.

The lower part of Table 3 indicates the focal length of objective lens5, the diameter of luminous flux incident on the objective lens 5, andthe numerical aperture of objective lens 5 in each of the cases whereDVD and CD-R are set as the optical recording medium.

Example 4

The objective lens 5 in Example 4 comprises, successively from the lightsource side, a biconvex lens 1 c having a surface constituted by anaspheric surface directed onto the light source side, and a negativemeniscus lens 2 c having a convex surface constituted by an asphericsurface directed onto the optical recording media side, which arecemented together. Here, the cemented face is a spherical surface.

The effects of this Example are substantially the same as those ofExample 2.

The upper part of Table 4 (follows) shows lens data (radius of curvatureR, surface space D, and refractive index N with respect to λ=650 nm and780 nm) of the objective lens 5 in accordance with Example 4.

The middle part of Table 4 indicates the aspheric surface coefficient ofeach aspheric surface.

The lower part of Table 4 indicates the focal length of objective lens5, the diameter of luminous flux incident on the objective lens 5, andthe numerical aperture of objective lens 5 in each of the cases whereDVD and CD-R are set as the optical recording medium.

Example 5

The objective lens 5 in Example 5 comprises, successively from the lightsource side, a negative meniscus lens 2 d having a convex surfaceconstituted by an aspheric surface directed onto the light source side,a biconvex lens 1 d having a surface with a stronger curvature directedonto the light source side, and a negative meniscus lens 3 d having aconvex surface constituted by an aspheric surface directed onto theoptical recording media side, which are cemented together. Each of thecemented faces is a spherical surface.

The upper part of Table 5 (follows) shows lens data (radius of curvatureR, surface space D, and refractive index N with respect to λ=650 nm and780 nm) of the objective lens 5 in accordance with Example 5.

As can be seen from Table 5, the meniscus lens 2 d, biconvex lens 1 d,and meniscus lens 3 d are configured such that their respectiverefractive indices with respect to wavelengths of 650 nm (λ₁) and 780 nm(λ₂) satisfy the following conditional expressions (7) and (10):

N_(1λ1)>N_(2λ1) and N_(1λ2)<N_(2λ2)  (7)

N_(2λ1)<N_(3λ1) and N_(2λ2)>N_(3λ2)  (10)

where

N₁ _(^(λ)) ₁ is the refractive index of the meniscus lens 2 d (the firstlens from the light source side) with respect to the light beam havingthe wavelength λ₁;

N₂ _(^(λ)) ₁ is the refractive index of the biconvex lens 1 d (thesecond lens from the light source side) with respect to the light beamhaving the wavelength λ₁;

N₃ _(^(λ)) ₁ is the refractive index of the meniscus lens 3 d (the thirdlens from the light source side) with respect to the light beam havingthe wavelength λ₁;

N₁ _(^(λ)) ₂ is the refractive index of the meniscus lens 2 d (the firstlens from the light source side) with respect to the light beam havingthe wavelength λ₂;

N₂ _(^(λ)) ₂ is the refractive index of the biconvex lens 1 d (thesecond lens from the light source side) with respect to the light beamhaving the wavelength λ₂; and

N₃ _(^(λ)) ₂ is the refractive index of the meniscus lens 3 d (the thirdlens from the light source side) with respect to the light beam havingthe wavelength λ₂.

Further, this Example satisfies the following conditional expressions(11) and (12):

N_(1λ1)=N_(3λ1)  (11)

 N_(1λ2)=N_(3λ2)  (12)

The effects obtained by this Example are considered similar to thoseobtained when Examples 3 and 4, each constituted by two lenses, arecombined together. Further, as mentioned above, the first and thirdlenses from the light source side have the same refractive index, sothat they can be formed from the same material, whereby they can be madeeasily at a lower cost.

The first and third lenses from the light source side may haverespective refractive indices different from each other with respect toeach of the wavelengths λ₁ and λ₂ as a matter of course.

The middle part of Table 5 indicates the aspheric surface coefficient ofeach aspheric surface.

The lower part of Table 5 indicates the focal length of objective lens5, the diameter of luminous flux incident on the objective lens 5, andthe numerical aperture of objective lens 5 in each of the cases whereDVD and CD-R are set as the optical recording medium.

FIGS. 6A to 10B are wavefront aberration charts for Examples 1 to 5mentioned above. From these charts, it is seen that wavefrontaberrations for both DVD and CD-R are made favorable.

The objective lens of the present invention can be modified in variousmanners without being restricted to that of the above-mentionedembodiment. For example, a plastic material can be used as a materialfor forming the lenses. Further, a resin curable by heat or light may bemolded into a desirable form while being attached to one lens, whereby athin lens cemented to one lens can be formed.

If the performance up to about NA=0.6 is secured for light having awavelength of 780 nm (λ₂), for example, when determining the form ofcemented lens surface, then a specific stop (such as a liquid crystalshutter, a wavelength-selective filter, or the like) for adjusting theNA of DVD and CD-R is unnecessary.

The optical recording media to be recorded and reproduced in the opticalpickup apparatus of the present invention are not restricted to DVD andCD-R, whereby the present invention is applicable to any case where asingle optical pickup apparatus is used for recording and reproducingtwo optical recording media which are different from each other in termsof the wavelength regions in use and/or thickness of the opticalrecording media.

Though the optical pickup apparatus in accordance with theabove-mentioned embodiment is provided with individual light sources foroutputting respective wavelengths of light different from each other, asingle light source which can output two wavelengths of light differentfrom each other may be provided instead.

Also, though λ₁<λ₂ in the above-mentioned embodiment, it can also beλ₂>λ₁ as a matter of course.

As explained in the foregoing, the objective lens for optical recordingmedia in accordance with the present invention and the optical pickupapparatus using the same utilize a property of a cemented face, suchthat, when light beams to be used have different wavelengths dependingon the thickness of discs, the cemented face exhibits a negative (orpositive) refracting power and a positive (or negative) refracting powerwith respect to light beams having wavelengths λ₁ and λ₂, respectively,according to the wavelength-dependent difference in refractive index ofmaterials constituting the lenses.

As the curvature of the cemented face is enhanced, the sphericalaberration can be changed in the positive (or negative) direction withrespect to light having the wavelength λ₁ and in the negative (orpositive) direction with respect to light having the wavelength λ₂.Thus, lens design parameters can exist such that the sphericalaberration changes in opposite directions depending on whether arelatively thick disc or relatively thin disc is used, thereby making itpossible to easily construct an objective lens which canrecord/reproduce discs having thicknesses different from each other by asingle lens.

Therefore, unlike the conventional objective lens for optical recordingmedia and optical pickup apparatus, there is no fear of the apparatusincreasing its size, complicating its structure, and raising itsmanufacturing cost in order to ameliorate its aberration.

TABLE 1 Radius of Surface Refractive Index (N) Lens Surface Curvature(R) Space (D) λ = 650 nm λ = 780 nm 1(Aspheric)  2.66660 0.10 1.617611.59876 2(Aspheric)  1.97610 3.10 1.60702 1.60008 3(Aspheric) −6.612801.00000 1.00000

When the lens surface is an aspheric surface, the radius of curvature(R) indicates the radius of curvature near the optical axis.

Aspheric Coefficients (1st Surface)

A2  3.027807920 × 10⁻³ A3 −2.678026024 × 10⁻⁵ A4  7.590256804 × 10⁻⁶ A5−7.414318848 × 10⁻⁶ K  0.0

Aspheric Coefficients (2nd Surface)

A2  1.290766314 × 10⁻² A3 −8.941446210 × 10⁻⁴ A4 −2.734198317 × 10⁻⁶ A5−5.706310400 × 10⁻⁸ K  1.000000000

Aspheric Coefficients (3rd Surface)

A2  1.310888519 × 10⁻² A3 −4.927184916 × 10⁻³ A4  8.234380791 × 10⁻⁴ A5−5.883457008 × 10⁻⁵ K  0.0

DVD

Focal length f=3.614 mm

Incident luminous flux diameter φ=4.34 mm

Numerical aperture NA=0.60

CD-R

Focal length f=3.635 mm

Incident luminous flux diameter φ=3.27 mm

Numerical aperture NA=0.45

TABLE 2 Radius of Surface Refractive Index (N) Lens Surface Curvature(R) Space (D) λ = 650 nm λ = 780 nm 1(Aspheric)  2.66660 3.00 1.607021.60008 2(Aspheric) −2.03340 0.20 1.61761 1.59876 3(Aspheric) −6.612801.00000 1.00000

When the lens surface is an aspheric surface, the radius of curvature(R) indicates the radius of curvature near the optical axis.

Aspheric Coefficients(1st Surface)

A2  2.890149036 × 10⁻³ A3 −5.183197858 × 10⁻⁵ A4  7.730867431 × 10⁻⁶ A5−8.318471494 × 10⁻⁶ K  0.0

Aspheric Coefficients(2nd Surface)

A2 −6.333291121 × 10⁻³ A3  2.713520032 × 10⁻³ A4 −2.384937508 × 10⁻⁴ A5−2.295935280 × 10⁻⁷ K −4.329785326 × 10⁻¹

Aspheric Coefficients(3rd Surface)

A2  1.247164921 × 10⁻² A3 −4.894763831 × 10⁻³ A4  8.265848030 × 10⁻⁴ A5−5.954487753 × 10⁻⁵ K  0.0

DVD

Focal length f=3.626 mm

Incident luminous flux diameter φ=4.35 mm

Numerical aperture NA=0.60

CD-R

Focal length f=3.634 mm

Incident luminous flux diameter φ=3.27 mm

Numerical aperture NA=0.45

TABLE 3 Radius of Surface Refractive Index (N) Lens Surface Curvature(R) Space (D) λ = 650 nm λ = 780 nm 1(Aspheric) 2.91060 0.10 1.718971.69680 2(Spheric) 2.13220 3.10 1.70202 1.69895 3(Aspheric) −10.11021.00000 1.00000

When the lens surface is an aspheric surface, the radius of curvature(R) indicates the radius of curvature near the optical axis.

Aspheric Coefficients (1st Surface)

A2  2.237489116 × 10⁻³ A3 −7.233666142 × 10⁻⁵ A4  4.692685600 × 10⁻⁷ A5−5.810314248 × 10⁻⁶ K  0.0

Aspheric Coefficients (3rd Surface

A2  6.418904979 × 10⁻³ A3 −3.438843543 × 10⁻³ A4  5.734468432 × 10⁻⁴ A5−3.855619631 × 10⁻⁵ K  0.0

DVD

Focal length f=3.605 mm

Incident luminous flux diameter φ=4.33 mm

Numerical aperture NA=0.60

CD-R

Focal length f=3.594 mm

Incident luminous flux diameter φ=3.23 mm

Numerical aperture NA=0.45

TABLE 4 Radius of Surface Refractive Index (N) Lens Surface Curvature(R) Space (D) λ = 650 nm λ = 780 nm 1(Aspheric) 2.91060 3.00 1.702021.69895 2(Spherical) −3.49650 0.20 1.71897 1.69680 3(Aspheric) −10.11021.00000 1.00000

When the lens surface is an aspheric surface, the radius of curvature(R) indicates the radius of curvature near the optical axis.

Aspheric Coefficients (1st Surface)

A2  2.200886162 × 10⁻³ A3 −7.331349972 × 10⁻⁵ A4 −5.992016310 × 10⁻⁷ A5−5.644264632 × 10⁻⁶ K  0.0

Aspheric Coefficients (3rd Surface)

A2  6.555913398 × 10⁻³ A3 −3.388259075 × 10⁻³ A4  5.694474379 × 10⁻⁴ A5−3.880251819 × 10⁻⁵ K  0.0

DVD

Focal length f=3.606 mm

Incident luminous flux diameter φ=4.33 mm

Numerical aperture NA=0.60

CD-R

Focal length f=3.594 mm

Incident luminous flux diameter φ=3.23 mm

Numerical aperture NA=0.45

TABLE 5 Radius of Surface Refractive Index (N) Lens Surface Curvature(R) Space (D) λ = 650 nm λ = 780 nm 1(Aspheric)  2.65090 0.10 1.617611.59876 2(Spherical)  2.33460 2.90 1.60702 1.60008 3(Spherical) −3.341100.20 1.61761 1.59876 4(Aspheric) −6.61280 1.00000 1.00000

When the lens surface is an aspheric surface, the radius of curvature(R) indicates the radius of curvature near the optical axis.

Aspheric Coefficients (1st Surface)

A2  3.002024251 × 10⁻³ A3 −4.479052221 × 10⁻⁵ A4  9.071357422 × 10⁻⁶ A5−8.541161981 × 10⁻⁶ K  0.0

Aspheric Coefficients (4th Surface)

A2  1.272616937 × 10⁻² A3 −5.067044944 × 10⁻³ A4  8.661727278 × 10⁻⁴ A5−6.301100124 × 10⁻⁵ K  0.0

DVD

Focal length f=3.602 mm

Incident luminous flux diameter φ=4.32 mm

Numerical aperture NA=0.60

CD-R

Focal length f=3.621 mm

Incident luminous flux diameter φ=3.26 mm

Numerical aperture NA=0.45

What is claimed is:
 1. An objective lens for optical recording media,said objective lens being constituted by two lenses cemented togetherand used for recording or reproducing two kinds of optical recordingmedia having thicknesses different from each other with two light beamshaving wavelengths different from each other, respectively; saidobjective lens satisfying the following conditional expression (1) withrespect to a light beam having a wavelength λ₁ used for recording orreproducing one of said two optical recording media, and the followingconditional expression (2) with respect to a light beam having awavelength λ₂ used for recording or reproducing the other of said twooptical recording media: N_(Lλ1)>N_(Dλ1)  (1) N_(Lλ2)<N_(Dλ2)  (2) where N_(Lλ1) is the refractive index of the lens on a light sourceside with respect to the light beam having the wavelength λ₁; N_(Dλ1) isthe refractive index of the lens on the optical recording media sidewith respect to the light beam having the wavelength λ₁; N_(Lλ) ₂ is therefractive index of the lens on the light source side with respect tothe light beam having the wavelength λ₂; and N_(Dλ) ₂ is the refractiveindex of the lens on the optical recording media side with respect tothe light beam having the wavelength λ₂.
 2. An objective lens foroptical recording media according to claim 1, wherein one of said lightbeams having wavelengths λ₁ and λ₂ is a light beam having a wavelengthof 650 nm used for recording or reproducing DVD, whereas the other is alight beam having a wavelength of 780 nm used for recording orreproducing CD-R.
 3. An objective lens for optical recording mediaaccording to claim 1, wherein said cemented face of lenses is anaspheric surface.
 4. An optical pickup apparatus comprising theobjective lens for optical recording media according to claim
 1. 5. Anobjective lens for optical recording media, said objective lens beingconstituted by two lenses cemented together and used for recording orreproducing two kinds of optical recording media having thicknessesdifferent from each other with two light beams having wavelengthsdifferent from each other, respectively; said objective lens satisfyingthe following conditional expression (3) with respect to a light beamhaving a wavelength λ₁ used for recording or reproducing one of said twooptical recording media, and the following conditional expression (4)with respect to a light beam having a wavelength λ₂ used for recordingor reproducing the other of said two optical recording media:N_(Lλ1)<N_(Dλ1)  (3) N_(Lλ2)>N_(Dλ2)  (4)  where N_(Lλ1) is therefractive index of the lens on a light source side with respect to thelight beam having the wavelength λ₁; N_(Dλ1) is the refractive index ofthe lens on the optical recording media side with respect to the lightbeam having the wavelength λ₁; N_(Lλ2) is the refractive index of thelens on the light source side with respect to the light beam having thewavelength λ₂; and N_(Dλ2) is the refractive index of the lens on theoptical recording media side with respect to the light beam having thewavelength λ₂.
 6. An objective lens for optical recording mediaaccording to claim 5, wherein one of said light beams having wavelengthsλ₁ and λ₂ is a light beam having a wavelength of 650 nm used forrecording or reproducing DVD, whereas the other is a light beam having awavelength of 780 nm used for recording or reproducing CD-R.
 7. Anobjective lens for optical recording media according to claim 5, whereinsaid cemented face of lenses is an aspheric surface.
 8. An opticalpickup apparatus comprising the objective lens for optical recordingmedia according to claim
 5. 9. An objective lens for optical recordingmedia, said objective lens being constituted by two lenses cementedtogether and used for recording or reproducing two kinds of opticalrecording media having thicknesses different from each other with twolight beams having wavelengths different from each other, respectively;assuming that said optical recording medium recorded or reproduced witha light beam having a wavelength λ₁ and said optical recording mediumrecorded or reproduced with a light beam having a wavelength λ₂ haverespective thicknesses t₁ and t₂ satisfying the condition of t₁<t₂, saidobjective lens satisfying the following conditional expression (5) whensaid cemented face of said cemented lenses is convex toward a lightsource, and the following conditional expression (6) when said cementedface of said cemented lenses is convex toward said optical recordingmedia: N_(Lλ1)>N_(Dλ1) and N_(Lλ2)<N_(Dλ2)  (5) N_(Lλ1)<N_(Dλ1) andN_(Lλ2)>N_(Dλ2)  (6)  where N_(Lλ1) is the refractive index of the lenson the light source side with respect to the light beam having thewavelength λ₁; N_(Dλ1) is the refractive index of the lens on theoptical recording media side with respect to the light beam having thewavelength λ₁; N_(Lλ2) is the refractive index of the lens on the lightsource side with respect to the light beam having the wavelength λ₂; andN_(Dλ) ₂ is the refractive index of the lens on the optical recordingmedia side with respect to the light beam having the wavelength λ₂. 10.An objective lens for optical recording media according to claim 9,wherein one of said light beams having wavelengths λ₁ and λ₂ is a lightbeam having a wavelength of 650 nm used for recording or reproducingDVD, whereas the other is a light beam having a wavelength of 780 nmused for recording or reproducing CD-R.
 11. An objective lens foroptical recording media according to claim 9, wherein said cemented faceof lenses is an aspheric surface.
 12. An optical pickup apparatuscomprising the objective lens for optical recording media according toclaim
 9. 13. An objective lens for optical recording media, saidobjective lens being constituted by three lenses cemented together andused for recording or reproducing two kinds of optical recording mediahaving thicknesses different from each other with two light beams havingwavelengths different from each other, respectively; assuming that saidoptical recording medium recorded or reproduced with a light beam havinga wavelength λ₁ and said optical recording medium recorded or reproducedwith a light beam having a wavelength λ₂ have respective thicknesses t₁and t₂ satisfying the condition of t₁<t₂, said objective lens satisfyingthe following conditional expression (7) when the cemented face betweenthe first and second lenses from a light source side is convex towardsaid light source: N_(1λ1)>N_(2λ1) and N_(1λ2)<N_(2λ2)  (7) saidobjective lens satisfying the following conditional expression (8) whenthe cemented face between the first and second lenses from the lightsource side is convex toward said optical recording media:N_(1λ1)<N_(2λ1) and N_(1λ2)>N_(2λ2)  (8) said objective lens satisfyingthe following conditional expression (9) when the cemented face betweenthe second and third lenses from the light source side is convex towardsaid light source:  N_(2λ1)>N_(3λ1) and N_(2λ2)<N_(3λ2)  (9) saidobjective lens satisfying the following conditional expression (10) whenthe cemented face between the second and third lenses from the lightsource side is convex toward said optical recording media:N_(2λ1)<N_(3λ1) and N_(2λ2)>N_(3λ2)  (10)  where N_(1λ1) is therefractive index of the first lens from the light source side withrespect to the light beam having the wavelength λ₁; N_(2λ1) is therefractive index of the second lens from the light source side withrespect to the light beam having the wavelength λ₁; N_(3λ1) is therefractive index of the third lens from the light source side withrespect to the light beam having the wavelength λ₁; N_(1λ2) is therefractive index of the first lens from the light source side withrespect to the light beam having the wavelength λ₂; N_(2λ2) is therefractive index of the second lens from the light source side withrespect to the light beam having the wavelength λ₂; and N_(3λ2) is therefractive index of the third lens from the light source side withrespect to the light beam having the wavelength λ₂.
 14. An objectivelens for optical recording media according to claim 13, satisfying thefollowing conditional expressions (11) and (12): N_(1λ1)=N_(3λ1)  (11)N_(1λ2)=N_(3λ2)  (12).
 15. An objective lens for optical recording mediaaccording to claim 13, wherein said first and third lenses from saidlight source side are formed from the same material.
 16. An objectivelens for optical recording media according to claim 13, wherein one ofsaid light beams having wavelengths λ₁ and λ₂ is a light beam having awavelength of 650 nm used for recording or reproducing DVD, whereas theother is a light beam having a wavelength of 780 nm used for recordingor reproducing CD-R.
 17. An objective lens for optical recording mediaaccording to claim 13, wherein at least one of said cemented faces oflenses is an aspheric surface.
 18. An optical pickup apparatuscomprising the objective lens for optical recording media according toclaim 13.